1. Field of the Invention
This invention relates to the formation of metal or metal alloy products by using spray casting techniques. The metal or metal alloy products which are produced according to this invention have a minimum of porosity.
2. Description of Related Art
It is well known in the art that a metal or metal alloy can be made by casting, spray casting into a die or by spray casting onto a substrate to form a particular shape. Casting a metal into a desired shape can be achieved by several different techniques, for example, sand casting, die casting, centrifugal casting, shell molding or investment casting. Articles produced by these methods, however, may possess poor mechanical properties mainly as a result of relatively large grain sizes, structural weaknesses and defects arising from the casting process, e.g., shrinkage and segregation.
The formation of a particular shape by casting involves the casting of a metal or metal alloy as an ingot, followed by a hot working step, e.g., hot rolling, forging, pressing or extruding. The formation of the finished shape is usually completed by a cold working process, e.g., cold rolling, pressing, coining or spinning. In either case, semifinished products (i.e. plates and bars) often have to be manufactured before subsequent processing to produce finished articles.
In the formation of particular shapes by spray casting, complex shaped articles can be manufactured having mechanical properties generally superior to those articles produced by casting the shapes by the first method described above. However, porosity is a major problem in the formation of shapes by spray casting.
Spray casting molten metal into a desired shape is achieved by atomization of the molten metal into a spray which is collected on a suitable substrate or die. In the past, almost all products produced with this process required hot working because of the high degree of porosity in the finished product. The problem with porosity is a major concern in this process. The process can produce products of a controlled degree of porosity as shown in U.S. Pat. No. 3,826,301 to Brooks, Column 2, lines 55-60.
The use of powder metallurgy in the production of metal or metal alloy shapes is also well known. Metal powder can be compacted by the use of dies to produce a number of desirable shapes. The produced shapes may be further worked to obtain, as far as it is possible, the desired physical properties. However, one of the limitations to this process is that the final product tends to exhibit undesirable amounts of porosity. Thus, in order to remove the porosity, it has been proposed to subject the finished shape to cold and/or hot working.
A process has been proposed for the direct fabrication of metal shapes of long length and relatively thin cross section by spray casting. The process comprises depositing a plurality of coherent layers of metal onto a substrate by directing sprays of atomized particles of molten metal onto the substrate. Then, a single layer is formed, while the metal is at a temperature above its recrystallization temperature. The metal layer is usually hot worked to provide for improved physical properties. This process is disclosed in U.S. Pat. Nos. 3,670,400 and 4,579,168 to Singer and is particularly applicable to the production of strip. The Singer patents disclose that atomized aluminum can be spray cast onto a moving target, such as a steel belt and the spray formed strip, while still hot, removed and hot rolled to the desired gauge. In this process, metal strip thicknesses of up to about 0.5" may be produced, with the thickness generally ranging from about 0.01 to 0.375".
The Singer patent also states that the porosity of the deposit layers ranges from about 15% to about 20%. When porosity is greater than 15%, a finished product would require hot working before further cold working steps can be performed.
Another prior art method that attempts to deal with the problems of porosity is described in U.S. Pat. No. 3,826,301 to Brooks. The Brooks patent teaches the production of shaped precision metal articles from molten metals and alloys by spray casting onto a die contoured to the shape of the desired article. The method disclosed in Brooks comprises directing an atomized stream of molten metal or metal alloy onto a collecting member to form a deposit and then directly working the deposited material on the collecting member by means of a die to form the desired shape. The purpose of the working is to densify the metal deposit which is porous. This is brought out in Column 2, lines 50-61, of the Brooks patent. Brooks states that the forming operation is normally carried out as soon as the required mass of metal has been deposited onto the die or collecting member. The patent also states that the spray deposit can be cold formed after it has been cooled. Another process for producing elongated metal articles is disclosed in U.S. Pat. No. 4,114,251 to Southern et al. The Southern et al patent discloses a process for producing elongated metal articles by atomizing molten metal and collecting the atomized particles on a moving support. The collected particles are then consolidated by, for example, passing the metal through rolls to form an elongated metal strip.
Another well-known technique for producing a continuous strip of metal is shown in U.S. Pat. No. 3,576,207 to Grenfell. The Grenfell patent discloses a process for continuous casting of metal strip by imparting an electrostatic charge of at least 80,000 volts to a stream of molten metal. The stream of metal is then passed through a nozzle into an inert gas flow and allowed to atomize into a fine spray. The spray droplets are then collected on a receiving surface to form a layer of metal on the collecting member. This is followed by continuously stripping the layer of metal from the collecting member.
In almost all of the above disclosed prior art references, metal articles are produced as either strip, ingots, discs or other shapes, but porosity is a problem in the spray cast products.
Other patent publications relating to the spray casting process include United Kingdom Patent Application Nos. GB 2,172,827A and GB 2,172,900A, European Pat. Applications EP 0,225,732 and EP 0,225,080 and Patent Cooperation Treaty Patent Application WO 87/03012.
The present invention is directed to the process for spray casting a metal or metal alloy wherein the finished product has a minimum of porosity. Thus, the process described by the present invention should eliminate the requirement of hot working the spray cast product. The porosity expected in the spray cast products made in accordance with this invention should be less than 15% and, preferably, less than about 10% by volume.
The process of the instant invention utilizes the techniques taught in Brooks, U.S. Pat. Nos. 3,826,301 and 3,909,921, to atomize molten metal and to deposit the atomized metal onto a collecting member. In performing the instant process, particular care is taken to control the volume fraction of solid of the atomized metal or metal alloy particles as they deposit on the collecting member.
In accordance with the instant invention, a process and apparatus for spray casting a metal or metal alloy is provided wherein the porosity of the produced metal or metal alloy should be substantially minimized. At least one supply of metal or metal alloy is held in a molten state. Then, at least first and second streams of the molten metal or metal alloy are allowed to issue from the supply. Each of the first and second streams is atomized into respective first and second sprays of partially solid particles. Each of the first and second sprays is deposited onto a collecting member. The particles deposited onto the collecting member solidify into a desired shape. The collecting member moves in at least one desired direction. The second spray is arranged to deposit onto the collecting member downstream of the first spray in the desired direction. The first spray deposits onto the collecting member with a first volume fraction of solid. The second spray deposits onto the collecting member with a second volume fraction of solid. The second volume fraction of solid is greater than that of the first volume fraction of solid.
The process of the instant invention may be used to spray cast a metal or metal alloy, to form ingots, to coat articles and to form any desirable shape, especially strip. In a preferred method of the invention, the metal or metal alloy is formed by atomizing the streams of metal or metal alloy by directing respective gas flows at the streams. The temperatures of the gas flows are less than the temperature of the streams of metal or metal alloy.
The volume fraction of solid of the respective metal or metal alloy sprays is controlled by varying: the flow rate of the atomizing gas; and/or the temperature of the atomizing gas; and/or the temperature of the stream of metal or metal alloy; and/or the distance the spray travels to the collecting member; and/or the flow rate of the stream of molten metal or alloy.
Any suitable gas may be used to atomize the stream of molten metal, but preferably the gas is non-oxidizing and inert. For example, nitrogen or argon would be acceptable. However, if oxidation of the particles is not undesirable, compressed air can be used as an atomizing medium. The atomizing step in the process for the present invention will be consistent with that taught in U.S. Pat. No. 3,826,301 to Brooks, which patent is incorporated herein by reference.